Research On Inductive Power Transmission With Constant Currenr Or Voltage Output And Its Applications

The Inductive Power Transmission (IPT) technology can transfer the energy in a predefined distance via the alternating electromagnetic field without the direct electrical contact. Compared with the traditional wired power supply, IPT has many unique advantages with no use of cumbersome wires to reduce loss and save cost, both electrical and mechanical isolation to improve the safety and reliability, and friendly operation in harsh environments, such as water and mine etc. Thus, IPT has been widely used in many applications, such as consumer electronics, lighting, electric vehicle charging, etc. The IPT converter should provide the required driving current or voltage for the load and then improve the transmission efficiency.In order to transfer the active power efficiently, IPT systems should have the following three characteristics with 1. the required constant current or voltage output for load,2. the zero reactive power in circuit, i.e., the purely resistive input impedance and the zero phase angle (ZPA) between the input voltage and current, and 3. the soft switching for the switching components to reduce the stresses in circuit and then improve the whole efficiency. It is well known that the input impedance and output characteristics of an IPT system are changing with the operating frequency and load condition. So it is difficult to meet the above three requirements at the same time with a range of load. Currently, the ZPA is usually achieved by the variable frequency control. The required constant current or voltage output is realized by the front-end or cascading converter. However, the second converter increases the cost and loss, and variable frequency control may bring the bifurcation phenomenon in some load ranges, which will cause the system unstable.In view of the above problems, this paper tries to investigate the compensation circuits with which the IPT converter can achieve the above objectives simultaneously. Then the control and circuit design can be simplified. Here, four basic compensating circuits are analyzed in detail, which are Series-Series (SS), Series-Parallel (SP), Parallel-Series (PS), and Parallel-Parallel (PP) compensation circuits. With the input impedance and output load characteristics of these four compensation circuits, it is found that SS and PP can realize the input ZPA and the output constant current simultaneously, whilst SP and PS can achieve the input ZPA and the output constant voltage simultaneously. The soft switching can be achieved by properly adjusting the compensation parameters. In order to validate the above analysis, the LED lighting application and battery charging application are investigated here. The wireless LED lighting system was built with SS compensation to provide a constant driving current, and the efficiency is measured above 90%. As the output current is related to the contactless transformer parameters, the contactless transformer is difficult to design under the given output current and the specified air gap and the volume for the IPT transformer. The paper then proposed an iteration method to design the IPT transformer. In the lithium battery wireless charging occasions, lithium batteries have two charging modes of the first constant current and then constant voltage charging. This paper proposed the hybrid compensation structure with SS and PS, where SS realizes the constant current charging and then PS is switched on to realize the constant voltage charging. The experimental results in these two applications agree well with the theoretical analysis.